Air pressure system classification
The shape of the curves on the isobaric maps at different heights of the atmosphere are changing at any time, and can be summarized into several basic types: high pressure, low pressure, high pressure ridge, low pressure trough and saddle pressure area.
(1) Low pressure
The air pressure system with a central air pressure lower than the ambient pressure has a space isostatic surface shaped like a valley and appears as a set of closed curves on the map. According to White Bell's law, the airflow in the low pressure zone rotates counterclockwise in the northern hemisphere, called a cyclone. For the lower atmosphere, the ground friction not only reduces the wind speed, but also biases the wind direction to low pressure, which causes the airflow to flow and rise from the periphery to the low pressure center, so there are often cloud and rain weather in the low pressure area.
(2) High pressure
The air pressure system with a central air pressure higher than the ambient air pressure has a spatial isostatic surface shaped like a mountain peak, and the figure shows a set of closed curves. The airflow in the high-pressure zone rotates clockwise in the northern hemisphere, called the anti-cyclonic. The bottom atmosphere of the high-pressure area diverges outward, causing the high air flow to sink, so the weather in the high-pressure area is fine.
(3) Low pressure trough
The narrow air pressure region extending from the low pressure zone is similar to the low pressure zone.
(4) High pressure tank
The narrow pressure region extending from the high pressure zone has a space isostatic surface shaped like a ridge, and the weather characteristics near the high pressure ridge are similar to those of the high pressure.
(5) Saddle pressure zone
The middle zone of two high pressures or two low pressures has a space isostatic shape like a saddle. When the saddle pressure field is just established, the wind speed is small, the wind direction is variable, and the air pressure value is relatively stable, but it may occur violently soon. weather change.